Diarylide pigments represent an important class of coloring agents used primarily for the manufacture of printing inks. Pigment Yellow 12 is the most widely used member of this group for solvent-based publication gravure printing inks. The almost-exclusive use of P.Y. 12 for the production of solvent-based publication gravure printing inks is due not only to its economic value, but also to its susceptibility to various surface treatments which impart certain desirable application properties.
A typical surface treatment agent for diarylide pigments is an amine. British Patent 1,085,835 teaches that fatty primary amines may be used to treat Pigment Yellow 12 so as to result in a pigment allegedly having increased tinctorial strength, gloss, transparency, and reduced rheology. U.S. Pat. No. 4,515,639 describes the use of diamines to treat diarylide pigments so as to reduce the penetration of inks prepared therefrom into lower quality uncoated paper stock. There are numerous other patents which describe the use of amine treatments to achieve an improved Pigment Yellow 12 for the preparation of publication gravure printing inks, e.g. U.S. Pat. Nos. 4,463,770; 4,462,833; 4,220,473; 3,905,825; 3,827,902., 3,655,641; and 3,607,861.
Amine-treated pigments possess a major deficiency. The commercially-important types of publication gravure printing inks prepared from such treated pigments exhibit poor storage stability. This instability is manifested by a red-to-green shift in hue, and also by an increase in viscosity, cf. "Pigmentation Problems of Yellow Publication Gravure Inks", Defazet 30(11), pp.486-90, 1976; "Pigment Developments", Ink & Print Vol.2, pp. 16-18, June 1984. The shift in chroma causes poor color matching unless it is detected, in which case additional color adjustments must then be made at considerable cost. The increase in viscosity results in a substantial loss of strength due to the use of additional solvent which is required to adjust the ink to print viscosity. Since additional ink base must be utilized to obtain the desired print strength, a significant economic penalty results.
The magnitude of the storage instability problem varies based on a number of factors, including the type of ink vehicle utilized, the type of dispersion equipment utilized to prepare the ink, the ambient temperature and the duration of ink storage. However, losses in excess of 20% are not uncommon, and losses of greater than 30% have been reported.
It is known that the instability problem may be avoided by the utilization of untreated pigments. However, this practice has found little or no commercial acceptance inasmuch as such untreated pigments exhibit inferior rheological and coloristic properties. In contradistinction thereto, the present invention provides for diarylide pigments which permit the manufacture of storage stable printing inks previously obtainable only with untreated pigments while maintaining the excellent rheological and coloristic properties associated with amine-treated pigments.